Chapter 1: Introduction to Molecular Regulation and Signaling

Introduction Molecular biology has opened the doors to new ways to study embryology and to enhance our understanding of normal and abnormal development. Sequencing the human genome, together with creating techniques to investigate gene regulation at many levels of complexity, has taken embryology to the next level. Thus, from the anatomical to the biochemical to […]

Chapter 2: Gametogenesis

Primordial Germ Cells Development begins with fertilization, the process by which the male gamete, the sperm, and the female gamete, the oocyte, unite to give rise to a zygote. Gametes are derived from primordial germ cells (PGCs) that are formed in the epiblast during the second week and that move to the wall of the […]

Chapter 3: First Week of Development

Ovarian Cycle At puberty, the female begins to undergo regular monthly cycles. These sexual cycles are controlled by the hypothalamus. Gonadotropin-releasing hormone (GnRH), produced by the hypothalamus, acts on cells of the anterior lobe (adenohypophysis) of the pituitary gland, which in turn secrete gonadotropins. These hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), stimulate and […]

Chapter 4: Second Week of Development

Introduction This chapter gives a day-by-day account of the major events of the second week of development; however, embryos of the same fertilization age do not necessarily develop at the same rate. Indeed, considerable differences in rate of growth have been found even at these early stages of development. Day 8 At the eighth day […]

Chapter 5: Third Week of Development

Gastrulation: Formation of Embryonic Mesoderm and Endoderm The most characteristic event occurring during the third week of gestation is gastrulation, the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Gastrulation begins with formation of the primitive streak on the surface of the epiblast (Figs. 5.1 and 5.2A). Initially, the […]

Chapter 6: Third to Eighth Weeks

Introduction The embryonic period, or period of organogenesis, occurs from the third to the eighth weeks of development and is the time when each of the three germ layers, ectoderm, mesoderm, and endoderm, gives rise to a number of specific tissues and organs. By the end of the embryonic period, the main organ systems have […]

Chapter 7: The Gut Tube and the Body Cavities

A Tube on Top of a Tube During the third and fourth weeks the top layer (ectoderm) of the trilaminar embryonic disc forms the neural plate that rolls up into a tube to form the brain and spinal cord by the process called neurulation (see Chapter 6). Almost simultaneously, the ventral layer (endoderm) rolls down […]

Chapter 8: Third Month to Birth

Development of the Fetus The period from the beginning of the ninth week to birth is known as the fetal period. It is characterized by maturation of tissues and organs and rapid growth of the body. The length of the fetus is usually indicated as the crown-rump length (CRL) (sitting height) or as the crown-heel […]

Chapter 9: Birth Defects and Prenatal Diagnosis

Birth Defects Birth defect, congenital malformation, and congenital anomaly are synonymous terms used to describe structural, behavioral, functional, and metabolic disorders present at birth. Terms used to describe the study of these disorders are teratology (Gr. teratos; monster) and dysmorphology. Dysmorphologists are usually within a department of clinical genetics. Major structural anomalies occur in approximately […]

Chapter 10: The Axial Skeleton

Introduction The axial skeleton includes the skull, vertebral column, ribs, and sternum. In general, the skeletal system develops from paraxial and lateral plate (parietal layer) mesoderm and from neural crest. Paraxial mesoderm forms a segmented series of tissue blocks on each side of the neural tube, known as somitomeres in the head region and somites […]